use pathops utils to improve precision of cubic chopping in scan converter

src/core/SkGeometry.cpp

 /*
  * Copyright 2006 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkGeometry.h"
 #include "SkMatrix.h"
 #include "SkNx.h"
@@ skipping 898 matching lines @@
     if (dst) {
         if (count == 0) {
             memcpy(dst, src, 4 * sizeof(SkPoint));
         } else {
             SkChopCubicAt(src, dst, tValues, count);
         }
     }
     return count + 1;
 }
 
+#include "../pathops/SkPathOpsCubic.h"
+
+typedef int (SkDCubic::*InterceptProc)(double intercept, double roots[3]) const;
+
+static bool cubic_dchop_at_intercept(const SkPoint src[4], SkScalar intercept, SkPoint dst[7],
+                                     InterceptProc method) {
+    SkDCubic cubic;
+    double roots[3];
+    int count = (cubic.set(src).*method)(intercept, roots);
+    if (count > 0) {
+        SkDCubicPair pair = cubic.chopAt(roots[0]);
+        for (int i = 0; i < 7; ++i) {
+            dst[i] = pair.pts[i].asSkPoint();
+        }
+        return true;
+    }
+    return false;
+}
+
+bool SkChopMonoCubicAtY(SkPoint src[4], SkScalar y, SkPoint dst[7]) {
+    return cubic_dchop_at_intercept(src, y, dst, &SkDCubic::horizontalIntersect);
+}
+
+bool SkChopMonoCubicAtX(SkPoint src[4], SkScalar x, SkPoint dst[7]) {
+    return cubic_dchop_at_intercept(src, x, dst, &SkDCubic::verticalIntersect);
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 /*  Find t value for quadratic [a, b, c] = d.
     Return 0 if there is no solution within [0, 1)
 */
 static SkScalar quad_solve(SkScalar a, SkScalar b, SkScalar c, SkScalar d) {
     // At^2 + Bt + C = d
     SkScalar A = a - 2 * b + c;
     SkScalar B = 2 * (b - a);
     SkScalar C = a - d;
@@ skipping 621 matching lines @@
         matrix.preScale(SK_Scalar1, -SK_Scalar1);
     }
     if (userMatrix) {
         matrix.postConcat(*userMatrix);
     }
     for (int i = 0; i < conicCount; ++i) {
         matrix.mapPoints(dst[i].fPts, 3);
     }
     return conicCount;
 }